Intersymbol interference (ISI) is a hindrance to high-speed digital communication. Effective digital communication depends on a sharp transition between data pulses whereas pulse transitions “smear” into each other in communication channels having ISI, a phenomenon denoted as pulse dispersion. Pulse dispersion occurs because high-frequency components of the data pulses are attenuated by the transmission medium. At higher data rates, the interference can become such that data pulses cannot be accurately distinguished from one another, leading to unacceptably high error rates.
Equalizers combat pulse dispersion by partially canceling the high-frequency cutoff that occurs in the transmission medium. A feedforward equalizer performs this mitigation of ISI using a combination of signal samples. In contrast, a feedback equalizer mitigates ISI based upon a combination of past output decisions. A decision feedback equalizer (DFE) is a combination of both a feedforward and a feedback equalizer and typically provides greater ISI mitigation then either technique alone. FIG. 1 illustrates an exemplary DFE 10, which includes a feedforward equalizer portion 105 and a feedback equalizer portion 110 to equalize an input signal s(t). A slicer 115 operates on the combined outputs from equalizer portions 105 and 110 to output a current digital decision 120. The number of taps in equalizer portions 105 and 110 is arbitrary and is denoted as n and m, respectively.
It will be appreciated that a feedback loop (not illustrated) is required to control the adaptation of the coefficients employed in the taps. For example, the input signal to slicer 115 may be sampled and compared to delayed versions of the slicer input signal to generate an error signal. The coefficients, which may be represented by a vector C, are then adapted responsive to the correlation between the error signal and the corresponding signal samples. However, a problem arises in that for a blind-adaptive equalizer having feedforward taps, coefficient vectors C and −C are equally valid. Thus, the coefficients have a natural tendency to be flipped leading to a polarity inversion of the output. In other words, the feedback loop for the adaptation is such that what should be binary ones at the output of slicer 110 become binary zeroes, and vice versa.
Accordingly, there is a need in the art for equalizers having constrained coefficient adaptation.